High-speed communication systems can operate by encoding information (e.g., data) onto radio waves that typically propagate along paths in free space. There are various ways to encode information onto a radio wave, as is understood by those in the art, including but not limited to Quadrature Amplitude Modulation (QAM), which uses both amplitude and phase information. Various factors (such as defects, misalignments, and orientation issues in optical components at the transmitter and/or receiver) can distort the transmitted polarization states in a random manner putting some of the power of an intended transmitted polarization state, corresponding for example to a digital one, into the other polarization state, corresponding to an unintended digital zero, or causing a signal in one polarization state to interfere with that of another polarization. When power is received in both polarization states in a level sufficient to interfere with accurate transmissions of the ones and zeroes, crosstalk (also referred to herein as cross-channel interference) is said to be present.
In addition, some coding schemes to encode information on radio wave require high levels of isolation at the receiver between the two data streams to guarantee accurate reception (i.e., with a bit error rate of less than 10−9, one erroneous received bit per 1 billion transmitted bits).
At least some existing point-to-point communication systems use different types of polarizations, including orthogonal polarizations, to increase the data rate, and such systems typically correct for the above-described crosstalk or cross-channel interference digitally. At ultra-high data rates (>50 Gbps), the added computational burden resulting from correcting for cross-channel interference can become onerous, particularly on applications such as unmanned airborne platforms, where size, weight, and power are constrained. If communication platforms (including but not limited to unmanned airborne platforms, airborne communication, etc.) are in relative motion, use of circular polarization can be advantageous. Effective use of polarization in communications systems can require that the transmitter (sender of information) be aligned in some way with the receiver (recipient of information), but this can be challenging if one or both of the sender and receiver are in motion. Misalignment can add further to the cross-channel interference.
One way to achieve circular polarization is through the use of wave plates (also known as retarders). Wave plates can, in some instances, be used as optical devices used to change the type of polarization of a light wave that travels through the plate. Wave plates do this by retarding (or delaying) one component of polarization with respect to its orthogonal component.
For example, in the optical set of wavelengths, quarter-wave plates (also referred to as ¼ λ plate) can be used to convert linearly polarized light into circularly polarized light or (in some instances) elliptically polarized light (and vice versa). Linearly polarized light can be transformed into circularly polarized light, and vice versa, by orienting a linear polarizer and quarter wave plate in a predetermined orientation. For example, a quarter wave plate with its axes oriented at 45° to linear polarization produces circular polarization. Conversely, a circular polarization (which does not have a specific orientation), passing through quarter wave plate produces linear polarization at 45° to the wave plate's axis. If linearly polarized light enters a quarter-wave plate at any angle besides 45°, the light becomes elliptically polarized. Thus, in an example communications system, having a transmitter and receiver, quarter wave plates can be used to convert linear to circular polarization at the transmitter, and back to linear at the receiver. This is but one way to convert linear to circular polarization.
In implementations that use millimeter wavelengths, linear-to-circular polarizers (referred to simply as “polarizers”) converts incident linear polarization from a first side to circular polarization on a second side, and incident circular polarization on a second side to linear polarization on a first side